HEAT-STABILIZED COMPRESSED 

WOOD (Staypak) 

Information Reviewed and Reaffirmed 

March 1956 

No. 1580 
HEAT-STABILIZED COMPRESSED WOOD (STAYPAK )1 

By R. M. SEBORG, Chemist 

M. A. MILLETT, Chemist 

and A. J. STAMM, Chemist 

Forest Products Laboratory, 2 Forest Service 

U. S. Department of Agriculture 

Introduction 

By compressing wood under conditions that cause sufficient flow of the lignin 

(the cementing material between fibers) to relieve the internal stresses resulting from the Compression, it has been found possible to eliminate the springback tendency encountered in the usual densified wood. When wood is compressed 

at higher temperatures and moisture contents than are normally used, it does 

not, when swollen, tend to recover the original uncompressed dimensions. Wood 

so processed has been named "staypak" by the Forest Products Laboratory. The 

process is covered by U. S. Patent 2,453,679 which is dedicated to the free 

use of the public in the United States (15). 

Stay-pak has distinct advantages over both the normal densified wood,

3 or "improved wood" as it is sometimes called, and compreg.

4 Whereas densified wood 

tends to lose its compression to a marked degree under conditions that cause 

swelling, staypak, although it will swell appreciably, will return to practically the original compressed thickness on drying to the original moisture 

content. 

1"Staypak" is heat-stabilized, compressed wood that has been heated, during 

the pressing process, under conditions such that the compression is not lost 

when the wood is subsequently swollen. Although it Will swell appreciably, 

it will return to practically the original compressed thickness on drying to 

the original moisture content. It is thus wood that "stays compressed" (12). 

2 Maintained at Madison 5, Wis., in cooperation with the University of 

Wisconsin. 

3Densified wood is normal untreated wood that has been compressed under conditions that do not cause flow of the lignin cementing material. It has been 

made from solid wood (4, 6, 7, 10, 11) and also from veneer preferably 

assembled with a synthetic resin glue (2, 5, 6, 7, 9, 16). 

4 Compreg is resin-treated compressed wood which is stabilized in the compressed form by the resin (3, 13, 14). 

Rept. No. 1580 (Revised) -1-
The behavior of compreg with respect to recovery of its original dimensions 

varies depending upon the conditions under which it was manufactured. Because 

of the stabilizing influence of the resin, it is possible to make compreg with 

negligible springback tendencies. The advantages of staypak over all compreg, 

however, lie in its strength characteristics. Although the shear strength 

parallel to the grain and in a plane perpendicular to the direction of compression of the wood is appreciably lower for staypak than for compreg, the 

impact strength is about double, and the tensile and bending strengths are 

approximately 25 percent more than those of compreg. 

The manufacturing conditions under which staypak with the optimum dimensional 

stability can be made will be discussed, together with the characteristics of 

the material thereby obtained, the species and form of wood that may be used, 

and the possible uses of the final product. Figures 1 through 7 show some of 

the properties of staypak. Figure 8 compares Izod-impact breaks of staypak 

with compreg . 

Effect of Moisture Content temperature-time 

Variables in Making Staypak 

A series of tests were made at the Forest Products Laboratory on yellow-poplar 

veneer to determine the best combinations of moisture content, temperature and 

time of pressing to obtain panels with optimum stability. Yellow-poplar was 

used because it is compressed to a greater. extent in making a high-specificgravity product than are denser hardwoods, and consequently its tendency to 

recover from compression should be greater. The choice of yellow-poplar, 

therefore, should accentuate differences in the product made under different 

conditions. Compressed parallel-laminated panels 13.5 by 9.5 inches by about 

0.6 inch were made from 21 plies of 1/16-inch veneer glued with Bakelite resin 

XC7381. All the panels were compressed at 2,000 pounds per square inch and at 

temperatures ranging from 300° to 360° F., using veneer conditioned-at 30, 50, 

and 65 percent relative humidity (moisture contents of 6, 9, and 12 percent, 

respectively) The time that the center of the panel was held at the heating 

temperature was either 5 or 30 minutes, The results of these tests are given 

in figures 1 to 4, inclusive. 

Figure 1 shows the relationship between the specific gravity to which the wood 

is compressed and the moisture content, temperature, and time of heating. The 

wood conditioned at 65 percent relative humidity compresses more readily than 

the wood conditioned at 30 percent relative humidity especially at the lower 

temperatures and shorter times, as a result of the plasticizing action of the 

additional moisture. An increase in temperature for the wood conditioned at 

the higher-relative humidity has less effect upon increasing the compressibility than the same temperature increase for wood conditioned at the lower 

relative humidity. The wood conditioned at higher relative humidity is 

sufficiently plasticized even at the lower temperatures to be almost completely compressed. An increase in the time has a similar effect. Under 

temperature conditions which do not cause optimum flow, increasing the time 

makes possible a greater extent of flow.

Rept. No. 1580 (Revised) -2-
Figure 2 shows that the water absorption, determined on 3- by 1- by 3/8-inch 

specimens (1 inch in the fiber direction) according to the Army Air Forces 

method (1) (weight increase after 24 hours' immersion), is much less for the 

specimens made from veneer conditioned at 65 percent relative humidity than 

for specimens made from veneer conditioned at 30 percent relative humidity, 

except at the highest temperatures. Increasing the time of heating for the 

less moisture-resistant specimens also greatly decreases the water absorption. 

A similar decrease in the equilibrium swelling and recovery from compression, 

and in the permanent recovery from compression after drying, occurs With increasing moisture content and temperature and time of heating (figs. 3 and 4). 

These two tests were made on cross sections 1/8-inch in the fiber direction 

that were immersed in water for 40 hours. The equilibrium swelling (fig. 3) 

under conditions such that the recovery is small (values for material heated 

at 350° F. and above) is about 25 percent. The reason why this swelling is 

considerably greater than the swelling of normal wood is that the wood still 

retains its normal swelling tendency but, because the volume has been reduced 

to about one-third of normal, the swelling per unit of reduced volume is about 

trebled. This increased swelling is not so serious as may be thought, for 

swelling of sizable pieces of highly compressed wood in the absence of a 

tendency to recover to the uncompressed dimensions, is extremely slow. Even 

in the small specimens of figure 7 the absorption of water is slow when the 

wood is properly stabilized. Both the rate and the total amount of dimension 

change occurring with time are considerably less than for the nonstabilized 

forms of compreg on the market (13). Staypak swells so slowly in sizable 

pieces that dimension changes and warping are negligible for most uses. The 

surface fibers of staypak, however, respond to humidity changes readily and, 

as a result, staypak weathers on the surface like normal wood, assuming a 

grayish color with many fine hair-line checks when exposed to direct sunlight 

and moisture changes for long periods of time without a surface finish. 

The wood is appreciably darkened in color under the stabilization conditions 

used in making staypak. The darkening is, undoubtedly, associated with some 

chemical change of the lignin that occurs under flow conditions. The color 

change is so sensitive that a series of specimens-of a single species arranged 

in the order of increasing darkness are in the order of increasing stability. 

Table 1 gives the properties of yellow-poplar staypak made from veneer containing 9 percent moisture (in equilibrium with 50 percent relative humidity) 

that was pressed at 2,000 pounds per square inch at a temperature of 340° F. 

for different periods of time. The data indicate a progressive improvement in 

the stability as the heating time is increased. Comparison of the data of 

figures 1 to 4, inclusive, Indicates that a 60-minute heating period gives 

practically optimum stability properties. 

The stabilization properties of 0.6 inch thick panels of staypak made from 

four species of wood, each at three moisture contents, together with the 

mechanical properties are presented in table 2. The equilibrium shrinkage 

between 9 and 30 percent relative humidities is given in place of the equilibrium swelling and recovery on water soaking. The measurements were made 

Rept. No. 1580 (Revised) - 3-
in the thickness direction of the panel on 1/8-inch cross sections, as were 

the recovery measurements. In all specimens the water absorption was equal 

to or less than the 6 percent allowed in the Army Air Forces specification of 

June 10, 1942 for compreg, but not quite equal to the value of 2.5 percent 

allowed in the revised specifications of March 15, 1944. 

Attempts to Plasticize Further the Lignin 

Attempts were made to plasticize further the lignin of the wood at the time of 

pressing by using yellow-poplar veneer previously treated with each of the 

following chemicals: 10 percent of urea, hexamethylene tetramine, and formaldehyde, followed by conditioning at 30 percent relative humidity and pressing 

at 350° F. for 15 minutes. The compression was not increased nor were the 

properties improved. Due to the hygroscopic properties of urea, it had an 

adverse effect upon the stability. Treating veneer containing 9 percent moisture with pyridine, an excellent swelling agent for lignin, also failed to 

give the expected increase in plasticity under stabilizing conditions. 

Attempts to make staypak with glycerine replacing the water plasticizer gave 

an inferior product. 

Veneer treated with a diacetone-alcohol solution of lignin so as to increase 

the lignin content of the wood by about 50 percent aided slightly in reducing 

the required compression pressure. The resulting staypak was darker in color 

and was more glossy than normal. The decrease in the required pressing 

pressure, however, was not sufficiently great to justify the treatment. It 

thus appears that the use of plasticizers other than water are not practical. 

Manufacturing Conditions 

Stabilization Temperature 

Although the data of figures 1 to 4, inclusive, and table 1 indicate that the 

use of a temperature of 350° to 360° F. would permit more rapid manufacture of 

stay-pak, experiments in making large panels of commercial size (54 by 14 by 

5/8 to 1-1/4 inches) indicate that these temperatures are too high when used 

on large pieces because they cause blistering and checking. This degrade in 

large, thick panels is due to the necessarily increased total time that the 

material must be in the press in order to get the heat to the center of the 

panels. The temperature range of 330° to 345° F. has proved to be the most 

satisfactory for making large panels of staypak that are check- and blisterfree, from birch and maple, using the lower temperature for the thicker 

material 

Moisture Content 

It is preferable to avoid moisture contents over 15 percent; it is still 

better to use veneer at 6 to 10 percent moisture content. 

Rept. No, 1580 (Revised) 
Excessive moisture (a) requires the use of excessive heat energy in raising 

the temperature of the wood to the required point. (b) increases the chances 

of the panel blistering or checking upon the release of the pressure, 

(c) increases the difficulty in properly bonding the plies together, and (d) 

gives a finished product that tends to lose moisture and shrink with time. As 

a moisture content of 8 percent may be considered to be a good average service 

equilibrium value for most pasts of the United States, all the developmental 

work on large panels of staypak has been confined to material conditioned at 

30 to 50 percent relative humidity to a moisture content of 6 to 9 percent, 

The wood has invariably been conditioned both before and after application of 

the bonding resin. It is believed, however, that the conditioning prior to 

application of the bonding resin can be omitted when the veneer is originally 

at approximately the desired moisture content. 

Subsequent exposure of the specimens to prolonged low relative humidity, as 

in heated, unhumidified buildings during the winter months, and heating in an 

unhumidified oven at 140° F. for 24 hours, showed that staypak made up at 

moisture contents appreciably above 6 percent tends to end check to some extent.. Staypak that is to be used in heated buildings or exposed to the direct 

rays of the sun under low relative humidity conditions should, hence, be made 

from wood conditioned to 6 percent moisture rather than the higher, more 

suitable values for rapid stabilization. 

Moisture Loss in Pressing 

One of the chief difficulties encountered in making staypak is the loss of 

moisture from the end grain of the veneer during pressing. If the final pressing temperature is attained before the wood is highly compressed, sufficient 

moisture is lost from the ends to prevent them from being properly stabilized. 

Insufficient stabilization at the ends of the panels can be detected by eye, 

for then the ends are not as dark in color as they are when properly stabilized. Under conditions such that an appreciable amount of moisture is lost 

from the end grain, an excessive part of the panel must be trimmed off before 

use. To avoid this loss of material and to make the process practical, a 

number of ways of avoiding; end-grain moisture loss were developed. The most 

effective of these is to load the press cold, apply the full compression pressure, and then heat to about 220° F. at the center of the panel with the 

platens at about 250° F. The wood is compressed very slightly at first, but 

as the temperature rises to 220° F. the compression becomes virtually complete. 

At this temperature the vapor pressure of water is only about 3 pounds per 

square inch in contrast to the vapor pressure of 120 pounds per square inch at 

350° F, The tendency for moisture to be driven out of the wood under pressure 

up to this stage is therefore relatively small. The compression that occurs 

at 220° F., however, is sufficiently complete to bottle the contained moisture 

within the wood. Subsequent heating of the wood to 330° to 350° F. after the 

compression at 220° F. is fairly complete causes the desired stabilization and 

completion of compression with a much smaller moisture loss than occurs when 

the wood is immediately raised to the higher temperature. 

Another aid, to the stabilization of the wood under heat and pressure, that 

tends to reduce moisture loss from the end grain of the surface plies, when 

Rept. No, 1580 (Revised) -5-
veneer is used, is to cut the two face plies about 1/4 inch shorter than the 

other plies. The assembly of plies is laid up for pressing with the face 

plies 1/8 inch shorter at each end than the rest of the assembly. This causes 

the second plies to spread over the end grain of the face plies when the assembly is compressed and, as a result, seal off the end grain capillaries of 

the face plies. Because of the greater heating and higher average temperature 

of the face plies during the compression stage of the process, the face plies 

are more subject to moisture loss than the inside plies. It is desirable to 

follow this technique in combination with the first method of avoiding moisture loss in making the stabilized, compressed product from veneer. 

The end grain moisture loss between the faces of the wood and the press 

platens or cauls may be minimized by a method that is effective for both 

veneer and solid wood. A strip of thermosetting glue about 1/2 inch wide is 

applied to the faces along the end grain. When the veneer assembly or the 

boards are pressed, this glue sets sufficiently to seal any capillaries formed 

between the wood and the press platens or cauls, thus preventing capillaries 

from carrying away water vapor. 

Pres sure 

The pressure of 2,000 pounds per square inch used in making yellow-poplar 

staypak (figs. 1 through 4, table 1), has been shown to be greater than 

necessary to make staypak under most stabilization conditions. Excessive 

pressure tends to cause a spreading of the panels that may reduce the shear 

strength at right angles to the direction of compression. Pressures of 1,400 

and 1,600 pounds per square inch appear to be adequate to give sufficient compression of yellow birch and sugar maple, respectively. 

When too low pressures are used, the product is far less dimensionally stable 

than when it is highly compressed. For this reason it is recommended that 

staypak always be compressed to a specific gravity of at least 1.30. 

Due to the thermoplastic nature of the lignin, it is important that the panels 

be cooled to somewhat below the boiling-point of water while under the full 

pressing pressure before they are withdrawn. 

Side Restraint 

When staypak is made from parallel-laminated veneer or from solid wood in 

thicknesses of 1/2 inch or more there is a tendency for the wood to spread in 

the across-the-fiber direction under pressure. When no side restraint is 

applied, the spreading of the panels is sufficient to reduce appreciably the 

specific gravity of the product near the edges. This same difficulty is encountered in the manufacture of compreg. Some form of side restraint should 

be used in making parallel-laminated staypak. This restraint can be provided 

by using a mold with stiff, sturdy sides capable of withstanding a side thrust 

of at least 500 pounds per square inch. 

Rept. No. 1580 (Revised) -6-
When a relatively few panels of each size and thickness are to be made, it 

Will not pay to have a mold made for this purpose. Blocks of wood or packs of 

veneer strips approximately equal in thickness to the original wood, placed 

snugly along the fiber-direction edges of the panel assembly will furnish the 

necessary restraint when pressure is applied. It is necessary that the veneer 

packs or blocks have their end grain against the panel for then the restraining material shows no tendency to spread in the same direction as the panels. 

Blocks need not be placed continuously along the edge of the panel to be 

pressed. Blocks 2 to 3 inches long in the fiber direction of the panel may be 

spaced 2 to 3 inches apart, thus applying an intermittent restraint along half 

the length of the panel. There is a slight spreading between restraining 

blocks, but this is not sufficient to reduce appreciably the specific gravity 

of the product along the edges of the panel. 

Time of Pressing 

It has been shown in figures 1 to 4, inclusive, that the stabilization time 

will vary with the stabilization temperature and �he moisture content of the 

wood. The time that the center of the wood need be held at the optimum temperature will also vary with the thickness of the wood. Thick material, such 

as 1-1/4-inch panels, will require a longer time to heat and to cool than 

7/8-inch panels, thus the center of the thicker material need not be held 

quite so long at the optimum temperature. Table 3 gives the time required to 

make 53- by 14-inch panels of birch staypak in two thicknesses from veneer 

conditioned at 50 percent relative humidity. The time required for heating 

and cooling the 7/8-inch staypak should be half that for the 1-1/4-inch 

staypak on the basis of the time varying as the square of the thickness. This 

was found to be approximately true. 

Type and Species of Wood 

The thickness of the veneer used has a negligible effect upon the properties 

of staypak. It is preferable from a manufacturing standpoint to use 1/8-inch 

veneer or even thicker when parallel-laminated products are sought because of 

the saving in handling costs and in glue. Staypak has been successfully made 

from solid wood. This material, which contains no synthetic resin, is comparable in properties with the parallel-laminated staypak made from veneer, 

showing that glue lines of synthetic resin do not improve the properties of 

the product . 

Tests were made to determined if cross-banded staypak can be made from preformed plywood. Matched panels of preformed plywood and plywood made up at 

the time of compressing and stabilizing, using a hot-setting phenolic glue, 

gave similar shear values in the plane of the plies indicating that no injury 

to the preformed synthetic resin glue lines occurs under the conditions for 

making staypak. Similar panels made with casein glue gave poor shear 

strengths in the preformed glue lines indicating that they will not stand the 

conditions for making staypak. Casein glue can, however, be used in making 

staypak from veneer when the glue is set at the time of compressing and 

stabilizing the wood. 

Rept. No. 1580 (Revised) -7-
Staypak can be made from a variety of species. It has been successfully made 

from veneer and solid wood of yellow birch, sugar maple, black walnut, and 

Sitka spruce, from veneer of yellow poplar, cottonwood, sweetgum, and Douglasfir, and from solid hickory, white oak, red alder, and white fir. Difficulty 

in stabilizing the product was encountered with solid Douglas-fir and 

ponderosa pine. This appeared to be due to the natural resins of the wood. 

Contrary to the findings withp other species, the stabilization was best at the 

ends of the specimens, presumably because the natural resins of the wood were 

squeezed out of the end grain. The fact that Douglas-fir veneer made satisfactory staypak while the solid wood did not was presumably due to the high 

temperature drying of the veneer that caused volatilization of part of the. 

natural resins and. fixing of the remaining resins, thus reducing the interferences of natural resins withP

stabilization. The only limitation in the 

choice of species for making staypak, according to the present procedure, seems 

to be avoiding the use of resinous, pitchy solid wood. 

It is surprising that oak, with its high acidity, was suitable for making 

staypak. Evidently the tendency for hydrolysis to take place under the manufacturing conditions is not great. 

When staypak is made of solid wood it should be made from flat-grain stock 

free from knots that are the full thickness of the wood. Summerwood will not 

compress to as great an extent as will springwood because of its high specific 

gravity. The summerwood rings will not compress to the degree that the wood 

as a whole will compress and hence they will buckle and distort badly when 

quarter-sawn boards are used, giving an unhomogeneous product. Knots have a 

considerably higher specific gravity than the normal wood and will take most 

of the load in compressing due to their lower compressibility. The knots will 

therefore tend to spread and crack the board. They are further liable to 

overstress the press. 

Strength Properties 

The strength properties of staypak are considerably higher than those of the 

normal wood from which it is made, the strength increases being about in 

direct proportion to the compression (table 2). The tensile strength, modulus 

of rupture, and modulus of elasticity in bending are significantly greater 

than for compreg of the same specific gravity. This is due to the fact that 

resin contributes to the weight and volume but not to these particular 

strength properties. Compreg, however, has a slightly higher compressive 

strength than staypak, due to the fact that resin does contribute to this 

property to a greater extent than to the increase in weight and volume. 

The chief advantage of staypak over adequately stabilized compreg is its 

superior impact strength. Staypak, in general, has twice the Izod values of 

compreg. The break across the specimen is fibrous in contrast to the granular 

break of compreg, indicating a lack of embrittlement in the staypak (fig. 8). 

Figures 5 and 6 show the notched toughness values for the specimens of yellowpoplar staypak of figures 1 to 4, inclusive. Figure 5 shows that staypak made 

from veneer in equilibrium with high relative humidity has somewhat higher 

Rept. No. 1580 (Revised) -8-
notched toughness values than the material made from veneer in equilibrium 

with lower relative humidity. This is due to the increased plasticization 

caused by increased moisture content, which results in an increased specific 

gravity under a fixed pressing pressure, as is shown by figures 1 and 6. 

Izod impact values vary considerably less with specific gravity than do the 

notched toughness values. It has been shown prior to this time (8) that at 

the highest specific gravities the notched-toughness values should be divided 

by 11.0 to convert to Izod values in foot-pounds per inch of notch, while at 

normal wood specific gravities the notched-toughness values should be divided 

by 6. The data indicate that both the notched toughness and the Izod values 

of compressed wood are independent of the degree of stability imparted to the 

product prior to any recovery of the unstabilized specimens. 

The shear strength parallel to the grain and in a plane perpendicular to the 

direction of compression of the wood (in the plane of the plies when veneer 

is used) is appreciably lower for staypak than for compreg. Any fibers that 

are ruptured during the pressing of compreg are rebonded with the resin 

within the plies. In staypak there is no resin present to do this. Unlike 

most properties, which do not seem to vary appreciably between species, the 

shear in the plane of the plies varies at least twofold between the species 

tested (table 2). A series of tests of the double cylindrical shear (1) 

parallel to the grain and in a plane perpendicular to the direction of compression gave average values of 3,700 pounds per square inch and 3,400 pounds 

per square inch for birch staypak (specific gravity 1.37) using 3/8-inch and 

1/2-inch diameter specimens, and 4,500 pounds per square inch and 3,900 

pounds per square inch for maple staypak (specific gravity 1.35) using 3/8­

inch and 1/2-inch diameter specimens, Where shear in the plane of the plies 

is important staypak made from maple or birch should be used. Where this 

shear value is not of great importance, the staypak can be made from more 

readily available species. 

A series of panels was prepared to determine if the high temperature to which 

the wood is heated in making staypak has any effect upon the shear in the 

plane of the plies. The data are given in table 4, segregated as to the 

position of the shear block in the panel and the position in the thickness 

direction of the blocks at which the shear tests were made. The fact that 

there is no consistent variation with temperature indicates that the lower 

shear strength of staypak as compared with compreg is due to the absence of 

resin within the plies rather than to the unusually high temperature of the 

process. The corresponding shear strengths for an uncompressed matched panel 

are also given in table 4. The shear strengths of the compressed staypak are 

all higher than for the uncompressed controls, even though they are not quite 

so high as for compreg. 

Maple staypak made from veneer conditioned at 30, 50, and 65 percent relative 

humidity (table 2) will meet all the Army Air Forces strength requirements 

for compreg (1) over the complete range of stabilization conditions tested. 

Birch, gum, and cottonwood staypak will meet all the strength requirements 

except the shear strength in the plane of the plies. In this property birch 

staypak will give borderline values, while gum and cottonwood will give 

values somewhat below the specified values 

Rept. No. 1580 (Revised) -9-
Gluing Staypak 

Preliminary gluing tests indicate that staypak can be glued to itself or to 

normal. wood more readily than can compreg. Good joints were obtained with 

various hot- and cold-setting urea and phenolic glues. 

Although good glue joints giving 100 percent wood failure can be obtained with 

small blocks that have not been urfaced, large thick panels are seldom sufficiently flat to give very good glue joints. A means of making the surfaces 

plane for gluing by scraping them with a special knife has been developed. 

Attack by Termites 

Preliminary termite exposure tests of birch staypak under severe outdoors conditions for 6 months indicate that staypak shows little if any resistance to 

attack and is considerably inferior to compreg in this respect. 

Potential Uses of Staypak 

Stay-pak should have many uses, for it has an Izod impact strength about twice 

that, of stabilized compreg and appreciably greater than that; of unstabilizedcompreg. Consequently it should be less susceptible to concentration of 

stresses, It has a greater stability than most of the high-impact-strength 

compregs, and it is potentially cheaper to produce. Service tests are being 

conducted on shuttles and picker sticks made of staypak for use in looms in 

textile mills. Because staypak is les notch sensitive than compreg it should 

be more suitable for spar plates and various fittings. It should also be 

suitable for tool handles, mallet head , pulleys, silent gears, and various 

tooling jigs and dies. Because of its greater swelling than that of compreg, 

staypak will not be so suitable for water-lubricated bearings or other underwater uses. 

Summary 

Solid flat-sawn nonresinous wood and veneer spread with glue and assembled in 

parallel-laminated or cross-banded form can be compressed to a high specific 

gravity product that does not tend to spring back under moisture absorbing 

conditions when it is pressed under conditions that cause some flow of the 

lignin. The conditions for making this material, called staypak, and its 

chief physical properties have been determined It is shown to have several 

strength properties superior to compreg, most notable of which is its greater 

impact strength. Although it is not as dimensionally stable as compreg, it 

is sufficiently stable for many uses. 

Rept. No. 1580 (Revised) -10 -
Literature Cited 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

Army Air Forces Specification, Wood: Impregnated Compressed, No. 15065, 

June 10, (1942). Revised No. 15065A, March 15, (1944). 

Bernhard, R. K., Perry, T. D., and Stern, E. G., Mechanical Engineering 

62: 189 (1940) . 

Burr, H. K. and Stamm, A. J., Forest Products Laboratory Report No. 1384, 

Jan. (1943). 

Esselen, Gustavus J., U. S. Patent No. 1,952,664 (1934). 

Finlayson, M., Transactions, Amer. Soc. Mech. Eng., Apr., 193 (1943). 

Gunn, A. L., Australian Timber Jour. 6(3):143 (1940). 

Kollman, F., "Technolgie des Holzes," Julius Springer, Berlin (1936). 

Millett, M. A., Seborg, R. M., and Stamm, A. J., Forest Products Laboratory Report No. 1386, May (1943). 

Olesheimer, L. J., U. S. Patent No. 1,707,135 (1929). 

Olsen, A, G., U. S. Patent No. 1,981,567 (1934). 

Sears, C. U., U. S. Patent No. 646,547 (1900). 

Seborg, R. M. and Stamm, A. J., Mechanical Engineering 63:211 (1941). 

Stamm, A. J. and Seborg, R. M., Transactions, Amer. Inst. Chem. Eng. 

37:385 (1941) . 

Stam, A. J. and Seborg, R. M., Forest Products Laboratory Report No. 

1381, Jan. (1942). 

Stamm, A. J., Seborg, R. M., and Millett, M. A., U. S. Patent 

2,453,679, Nov. (1948). 

Walsh, F. L. and Watts, R. L., U. S. Patent No. 1,465,383 (1923). 

Rept. No. 1580 (Revised) -11-
Table 1.--Effect of variations in the stabilization time of 

yellow-poplar staypak upon its properties1 

1Wood preconditioned at 50 percent relative humidity to 9 percent moisture 

content, pressed at 2,000 pounds per square inch, final. stabilization 

temperature 340° F., final thickness of panel 0.6 inch. 

Rept. No. 1580 (Revised) 
Table 2.--Properties of staypak made from veneers of four sperics conditioned a three relative humidities1 

Z M 52278 F 
Table 3.--Time for making 53- by 14-inch panels of yellow 

birch staypak of two different thicknesses from 

veneer conditioned at 50 percent relative 

humidity 

Rept. No. 1580 (Revised) 
ZM52279F 
Figure 1.--Effect of the relative humidity at which the veneer was conditioned, the final heating temperature, and the time upon the specific

gravity of parallel-laminated yellow-poplar staypak pressed at 2,000 

pounds per square inch. 

Figure 2.--Effect of the relative humidity at which the veneer was conditioned, the final heating temperature, and the time upon the water 

absorption of parallel-laminated yellow-poplar staypak pressed at 

2,000 pounds per square inch. 

ZM 52274 F 
Figure 3.--Effect of the relative humidity at which the veneer was conditioned, the final heating temperature, and the time upon the equilibrium 

swelling and recovery in the thickness direction of parallel-laminaled 

yellow-poplar staypak pressed at 2,000 pounds per square inch. 

Figure 4.--Effect of the relative humidity at which the veneer was conditioned, the final heating temperature, and the time upon the permanent 

recovery from compression, of parallel-laminated yellow-poplar staypak 

pressed at 2,000 pounds per square inch. 

Z M 52275 F 
Figure 5.--Effect of the relative humidity at which the veneer was conditioned, the final heating temperature, and the time upon the notched 

toughness of parallel-laminated yellow-poplar staypak pressed at 2,000 

pounds per square inch. 

Figure 6.--Data of figure 5 plotted to show that differences in notched 

toughness for panels made from veneer conditioned at different relative humidities are due to differences in the specific gravity to 

which the wood is compressed. 

Z M 58276 F 
Figure 7.--Rate of water absorption by birch staypak made from veneer conditioned at 50 percent relative humidity and pressed under different stabilizing conditions. The stabilizing time is the time 

that the center of the specimen is held at the optimum heating temperature. The specimens were 1 

by 0.75 by 3 inches, the 1-Inch dimensinn being in the fiber direction. 

Z M 52277 F 
Figure 8.--Comparison between the Izod impact breaks 

of compreg (left) and staypak (right) Z M 58906 F